As a light source of vehicle headlamps, longer-life, lower power consumption LEDs have been spread as a substitute for halogen lamps. On the other hand, as for headlamps using LEDs as a light source, since the amount of light emission of a single LED is still small, a plurality of LEDs are arranged in a block to be lit so as to maintain the amount of light emission required as the headlamps. Thus, since the amount of light emission of the individual LEDs is small, it can easily occur that even if a part of the LEDs of the headlamps has stopped, a driver does not become aware of that. Accordingly, a device is necessary which detects that a part of the LEDs of the headlamps is not lit and notifies the driver of that.
Patent Document 1 discloses a lighting apparatus that lights headlamps which employ a block arranged by connecting a plurality of LEDs in series as the light source, and detects that a part of the LEDs of the headlamps short-circuits and becomes abnormal. The apparatus measures the output voltage of the lighting apparatus and the voltage of a single LED in the block, and makes, if a relative value between them varies, a decision that a part of the plurality of LEDs becomes abnormal.
In addition, Patent Document 2 discloses a lighting apparatus that lights headlamps which employ a block arranged by connecting a plurality of LEDs in series as the light source. The apparatus captures a moment at which a change occurs in the output voltage of the lighting apparatus, and makes a decision that abnormality occurs because of a short circuit of a part of a plurality of LEDs.
As the foregoing documents offer as a problem, if the plurality of LEDs connected in series have variation in the forward voltage of the individual LEDs, even if the forward voltage of a single LED in the block of the series connection becomes zero volt because of a short circuit failure or becomes a Zener voltage of its parallel Zener diode because of an open failure, they are hidden under a variation tolerance of the output voltage of the LED lighting apparatus, and a decision of the failure cannot be made from only the output voltage. Although measuring the forward voltage of each of the plurality of LEDs constituting the LED block makes it possible to locate the failure, this is not reasonable because the configuration becomes complicated and a large number of measuring operations are very troublesome.
On the other hand, the forward voltage of the LEDs varies every moment in accordance with their energized duration and ambient temperature of travelling of a vehicle such as a temperature of an environment in which the headlamps are lit. Accordingly, the Patent Document 2, which employs only instantaneous voltage changes, has a problem of being unable to detect a failure accurately. In addition, the Patent Document 1, which monitors the forward voltage of a single LED to correct the changes in the forward voltages of the LEDs, has a problem of requiring new wiring for measuring the voltage and thus complicating the configuration.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide a headlamp LED lighting apparatus and a vehicle headlamp lighting system using the same, the apparatus being simple in construction and capable of positively detecting an abnormality occurring in a part of a plurality of LEDs in accordance with a change of the average of the output voltage detected at every prescribed interval in the vehicle headlamp that employs a block of a plurality of LEDs connected in series as a light source.